Reactions of dienes with the ion [Ir(PPh3)2(OCMe2)2H2]+ : a kinetic and mechanistic study of complex formation and diene hydrogenation

Abstract

Stopped-flow n.m.r. reveals markedly different rates of co-ordination of cyclo-octa-1,5-diene (cod) and bicyclo[2.2.1]hepta-2,5-diene (norbornadiene, nbd) to the ion [Ir(PPh₃)₂(OCMe₂)₂H₂]⁺(1). [Ir(PPh₃)₂(diene)H₂]⁺ ions are the first formed products. The interconversion of all three [Ir(PPh₃)₂(nbd)H₂]⁺ isomers has been observed by ¹H and ³¹P n.m.r. spectroscopy. These species react further to give [Ir(PPh₃)₂(nbd)]⁺(2), either by H₂ elimination, or by hydrogenation of the co-ordinated nbd to form bicyclo[2.2.1]hept-2-ene (norbornene, nbe). Stopped-flow spectrophotometry reveals a marked inverse kinetic isotope effect for the formation of (2) during the hydrogenation or deuteriation of nbd. A mechanism involving an equilibrium between diene–dihydride and alkyl–hydride is proposed. The dienes 2,3-dimethylbutadiene and cyclohexa-1,3- or -1,4-diene react with (1) to give highly fluxional allyl–hydride species which feature an aliphatic C–H ⋯ Ir^III interaction. Variable-temperature ¹H and ³¹P n.m.r., and ¹H spin-saturation transfer experiments reveal a facile reversible hydrogen-transfer process between metal and hydrocarbon ligand which occurs via a C–H ⋯ Ir^III interaction.